There are various kinds of metal oxides, which are used for a wide range of applications. Indium oxide is a well-known material and has been used for transparent electrodes required in liquid crystal display devices or the like.
Some metal oxides have semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics are, for example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such metal oxides is already known (for example, see Patent Documents 1 to 4, Non-Patent Document 1, and the like).
As metal oxides, not only single-component oxides but also multi-component oxides are known. For example, homologous compounds InGaO3(ZnO)m (m is a natural number) are known multi-component oxides containing In, Ga and Zn (also referred to as an In—Ga—Zn-based oxide) (e.g. see Non-Patent Documents 2 to 4 and the like).
Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel formation region of a thin film transistor (for example, see Patent Document 5, Non-Patent Documents 5 and 6, and the like).